For detoxification and/or degermination of fluids contaminated by pollutants or microorganisms, photochemical or photobiological processes are developed. In all these processes oxygen-rich species, such as singlet oxygen, hydroxyl or other oxygen-rich radicals or other strongly oxidizing intermediates, are produced by photonic excitation, which species effect a decomposition and/or deactivation of the pollutants and/or microorganisms.
Examples are the photonic activation of reagents, such as hydrogen peroxide or caroat, or photocatalytic processes, such as the semiconductor photocatalysis, using titanium dioxide, for example, or the light-amplified Fenton reaction (photo Fenton reaction).
For accomplishing this task, solar energy is used besides electrical light sources, such as gas discharge lamps, incandescent lamps, fluorescent lamps, light emitting diodes or tubes, excimer radiators and lasers. However, the non-solar light sources offer low efficiencies at moderate investment costs, which results in high costs for both the required electrical energy and cooling processes. Further, the illuminants are comparably expensive and have a short service life. The high illuminant temperatures produced during operation, the high electrical voltages and power as well as the frequently used toxic constituents, such as mercury vapor, further entail a high expenditure for safety equipment.
Besides the better sustainability of the light source mentioned last, financial incentives in addition to the ecological incentives may be an inducement to use solar energy in view of the comparably lower operating costs of solar plants.
Various receiver reactor concepts are presented concerning the use of the sun as a light source for the aforementioned fields of application.
In DE 198 44 037 A1 a flatbed receiver reactor for solar-photo and solar-thermochemical syntheses is described. In particular at high concentrations or high extinction coefficients of the dissolved substances or in the case of strongly turbid fluids, such as emulsions or suspensions, the use of such reactors with comparably thick fluid layers is disadvantageous. The penetration depth of the light into the reaction mixture is very small due to the light absorption according to the Lambert Beer law and due to light scattering at the particles and/or droplets.
Falling film reactors have been put to test with a view to solving this problem, among others (D. Bahneman, M. Meyer, U. Siemon, D. Mencke, A Self-Sufficient PV Powered Solar Detoxification Reactor for Polluted Waters, Proc. Int. Sol. Energy Conf. Solar Engineering—1997, Apr. 27-30, 1997, ASME, Washington D.C., 261-267; B. Braun, J. Ortner, K.-H. Funken, M. Schäfer, C. Schmitz, G. Horneck, M. Fasdni, Dye-Sensitized Solar Detoxification and Disinfection of Contaminated Water, Proc. 8th Int. Symp. Solar Thermal Concentrating Technologies, Vol. 3, C. F. Müller Verlag, Heidelberg (1997) 1391-1401). It is a drawback of the falling films that they require large covers which are expensive to produce. Further, much energy is consumed for repeatedly pumping the reaction mixture across the falling film surface. The use of falling films open to the atmosphere was demonstrated, but in this case low-boiling substances are uncontrolledly released into the atmosphere.
Multi-ribbed plate reactors which do not concentrate light, in particular double-ribbed plate reactors made of an extruded translucent plastic material (EP 0 738 686 A1) and so-called CPC reactors (compound parabolic collectors) (for example J. I. Ajona, A. Vidal, The Use of CPC collectors for Detoxification of Contaminated Water; Design, Construction and Preliminary Results, Solar Energy 68 (2000) 109-120), have been developed and put to test for solar detoxification of contaminated waste waters.
These reactor systems offer similar efficiencies (R. Dillert, R. Goslich, J. Dzengel, H.-W, Schuhmacher, D. Bahnemann, Field Studies of Solar Water Detoxification, Proc. 1st User Workshop Training and Mobility of Researchers' Program at Plataforma Solar de Almeria, Nov. 18-19, 1997, Almeria, Spain, Ser, Ponencias, Madrid (1998) 31-40). However, it has turned out that the multi-ribbed plate reactors tend to foul to a considerable extent, and that their efficiency rapidly decreases as compared with that of CPC reactors. CPC-reactors are disadvantageous in that they require a reflector of complex configuration. Therefore the investment costs for this reactor technology are comparably high. Further, the reflectors may be subject to optical or mechanical degradation.
Line- and point-focusing concentrators have also been used for solar detoxification (DE 434 41 63 A1). However, these concentrators only utilize the direct radiation and not the diffuse radiation of the sun which contains a particularly large amount of UV light. Although the light concentration leads to more rapid concentration changes than non-concentrated sunlight, the reactor-related solar photon yield is comparably small, and the treatment costs are correspondingly high due to the comparably high investment costs.
In a photoreactor comprising transparent tubes, as described in DE 100 09 060 A1 of the present applicant, similar decomposition rates as those of CPC collectors were obtained without a combination with reflectors. Here, too, the ratio of the aperture to the size of the inner surface of the tube is disadvantageous. A corresponding increase may result in a further enhancement of the reactor-related photon yield.
A general drawback of the prior art is the very small reactor-related solar photon yield. In particular the scattering losses, which occur when suspensions of finely dispersed photocatalysts on the basis of titanium dioxide, for example, are used, result in an inefficient utilization of the emitted photons in the prior-art solar reactor configurations.
The known solar tube reactors are further disadvantageous in that they are operated at a high velocity with a turbulent flow for preventing sedimentation of turbid substances and/or suspended photocatalysts.
The agitation of the entire fluid flow at a high velocity results in a relatively high energy consumption for driving pumps. It would be of advantage to the economic efficiency and the ecobalance of a solar photoreactor plant if only a smaller portion of the fluid had to be placed in turbulence.
A photoreactor described in U.S. Pat. No. 4,456,512 A comprises an inlet chamber provided with a fluid inlet, from which inlet chamber a bundle of capillary tubes extends. The capillary tubes extend through a room in which a plasma is produced. The outer wall of this room is cooled. In the reaction fluid passing through the capillary tubes photochemical reactions take place.
A similar photoreactor is described in U.S. Pat. No. 3,554,887. Here, the tubes are connected with an inlet chamber and an outlet chamber. Inside the tube system a light source is arranged which produces the light required for the photoreaction.